Monitoring Silane Sol-Gel Kinetics with In-Situ Optical Turbidity Scanning and Dynamic Light Scattering

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Article Monitoring Silane Sol-Gel Kinetics with In-Situ Optical Turbidity Scanning and Dynamic Light Scattering

Abul Bashar Mohammad Giasuddin and David W. Britt *

Department of Biological Engineering, Utah State University, Logan, UT 84322-4105, USA * Correspondence: [email protected]; Tel.: +1-435-787-2158

Received: 13 June 2019; Accepted: 2 August 2019; Published: 6 August 2019

Abstract: Organosilanes (e.g., R’-SiOR3) provide hydrophobic functionality in thin-film coatings, porous gels, and particles. Compared with tetraalkoxysilanes (SiOR4), organosilanes exhibit distinct reaction kinetics and assembly mechanisms arising from steric and electronic properties of the R’ group on the silicon atom. Here, the hydrolysis and condensation pathways of n-propyltrimethoxy silane (nPM) and a tri-fluorinated analog of nPM, 3,3,3-trifluoropropyl trimethoxy silane (3F), were investigated under aqueous conditions at pH 1.7, 2.0, 3.0, and 4.0. Prior to hydrolysis, 3F and nPM are insoluble in water and form a lens at the bottom (3F) or top (nPM) of the solutions. This phase separation was employed to follow reaction kinetics using a Turbiscan instrument to monitor hydrolysis through solubilization of the neat silane lens while simultaneously tracking condensation-induced turbidity throughout the bulk solution. Dynamic light scattering confirmed the silane condensation and particle aggregation processes reported by the turbidity scanning. Employing macroscopic phase separation of the starting reactants from the solvent further allows for control over the reaction kinetics, as the interfacial area can be readily controlled by reaction vessel geometry, namely by controlling the surface area to volume. In-situ turbidity scanning and dynamic light scattering revealed distinct reaction kinetics for nPM and 3F, attributable to the electron withdrawing and donating nature of the fluoro- and organo-side chains of 3F and nPM, respectively.

Keywords: organosilane; ﬂuorosilane; ORMOSIL; hydrophobic nanoparticle; aqueous sol-gel synthesis; Turbiscan

1. Introduction

Tri-alkoxysilanes, R’Si(OR)3, are widely employed to introduce functionality via the R’ chemistry into sol-gel processes, with specific applications as adhesion promoters and cross-linkers, as resin precursors, and to capture and encapsulate biological molecules [1,2]. For alkyl or aryl R’-groups, the resulting particles or gel are referred to as organically modified silicas (ORMOSILs). When fluorine is present on the R’ chain, they may be referred to as F-ORMOSILs. The hydrophobic alkyl- and fluoro R’ groups lower the surface energy of the resulting sol-gel based thin films, gels, or particles, and have been widely explored to create non-reflective, non-fouling, and superhydrophobic materials [3–11]. The hydrolysis and condensation chemical reactions generally occur through nucleophilic substitution reactions, with water attacking the Si center to liberate alcohol. Methoxy and ethoxy moieties are the two most common alkoxy leaving groups, and their hydrophobicity renders alkyl- and fluoro-silane precursors insoluble in aqueous solution. An R’ alkyl- or fluoro-group on the silane further reduces solubility, and an organic cosolvent, such as ethyl alcohol, is frequently employed to improve solubility and control reaction pathways. The sol-gel reactions for a trialkoxy ORMOSIL are depicted as:

Molecules 2019, 24, 2931; doi:10.3390/molecules24162931 www.mdpi.com/journal/molecules MoleculesMolecules 201 2019, 92, 42,4 ,x x FOR FOR PEERPEER REVIEW REVIEW 2 of 15 2 of 15 Molecules 2019, 24, 2931 2 of 14 employed to improve solubility and control reaction pathways. The sol-gel reactions for a trialkoxy employed to improve solubility and control reaction pathways. The sol-gel reactions for a trialkoxy ORMOSIL are depicted as: ORMOSIL are depicted as: R’Si(OR)R’Si(OR)33 ++ nHnH22OO R’Si(OR)R’Si(OR)3 n3(OH)-n(OH)n +n nROH+ nROH Hydrolysis Hydrolysis (1) (1) − R’Si(OR)R’Si-OR3 ++ HOnH-2SiR’O R’SiR’Si(OR)-O-SiR’ + 3ROH-n(OH) n + nROH A lcohol condensation Hydrolysis (2) (1) R’SiR’Si-OR-OH + HO+ HO-SiR’-SiR’ R’SiR’Si-O-SiR’-O- SiR’+ H2O + ROH Water c ondensation Alcohol condensation (3) (2) R’Si-OR + HO-SiR’ R’Si-O-SiR’ + ROH Alcohol condensation (2) R’Si-OHAt + aHO minimum,-SiR’ for complete R’Si hydrolysis-O-SiR’, three + H 2 molesO of water are Water required c ondensation per mole of (3) trialkoxysilane. Excess water, along with acid catalysis, favors full hydrolysis (030) prior to initiationAt a minimum, of condensation. for completeControlling hydrolysis the reaction, three rates molesof tri-alkoxysilane of water arehydrolysis required and per mole of R’Si-OH + HO-SiR’ R’Si-O-SiR’ + H2O Water condensation (3) trialkoxysilane.condensation affords Excess control water, over along the size with and acidmorphology catalysis, of polycondensation favors full hydrolysis products that (030) prior to initiationassembleAt a of minimum, into condensation. nanoparticles for complete (sol) Controlling th hydrolysis,at may undergo the three reaction growth moles/ ofaggregation rates water areof into requiredtri - largeralkoxysilane per particles mole and ofhydrolysis and condensationtrialkoxysilane.form gels depending affords Excess water, on control reaction along over with conditions, the acid catalysis,size such and as favors acid morphology or full base hydrolysis catalysis of (030) polycondensation, as prior illustrated to initiation in the products that reaction diagram in Figure 1. In the reaction scheme, (x,y,z) report the status of hydrolysis and assembleof condensation. into nanoparticles Controlling the reaction(sol) th ratesat may of tri-alkoxysilane undergo growth hydrolysis/aggregation and condensation into alargerﬀords particles and controlcondensation over the size and morphology of polycondensation products that assemble into nanoparticles form gels depending on reaction conditions, such as acid or base catalysis, as illustrated in the (sol) that may undergo growth/aggregation into larger particles and form gels depending on reaction reactionconditions, diagram such as in acid Figure or base 1. catalysis, In the as reaction illustrated scheme in the reaction, (x,y,z) diagram report in the Figure status1. In of the hydrolysis and condensationreaction scheme, (x,y,z) report the status of hydrolysis and condensation

X,Y,Z = (OR,OH,OSi)

Figure 1. Schematic of the reaction scheme of hydrolysis and condensation kinetics of trialkoxysilanes under acidic and basic conditions in aqueous media, along with a numerical reaction scheme [11]. Here, (300), (030), and (003) represent the -OR, -OH, and -OSi functionalities, respectively, around a single Si atom. The neat silane consists of three alkoxy groups (300), which under acidic conditions are sequentially hydrolyzed to three hydroxylX,Y,Z = (OR,OH,OSi) groups (030), which may subsequently condense to produce three siloxane bonds around the Si center (003). FigureFigure 1. 1.Schematic Schematic of the of reaction the scheme reaction of hydrolysis scheme and of condensation hydrolysis kinetics and of condensation tri-alkoxysilanes kinetics of tri- alkoxysilanesunderOn a acidic given and under Si basic atom, acidic conditions x indic andates in basic aqueous the conditions number media, of along in alkoxy aqueous with (- aOR) numerical media, leaving reactionalong groups with scheme present, a numerical [11]. y the reaction numberHere, of (300), (-OH) (030), groups, and (003) and represent z the number the -OR, of -OH, (-O- andSi) groups -OSi functionalities,, as depicted respectively, in the schematics. around a Thus, scheme [11]. Here, (300), (030), and (003) represent the -OR, -OH, and -OSi functionalities, for asingle trialkoxysilane Si atom. The under neat silane acid consists catalysis of three, the alkoxyprecur groupssor (300) (300), follows which a under stepwise acidic hydrolysis conditions are to form (210)respectively,sequentially to (120), and hydrolyzed around (030) aproducts. to single three hydroxyl Si Under atom. groups basic The (030), conditions,neat which silane may condensation consists subsequently of three pathways condense alkoxy to are produce groups promoted (300),, which yieldingunderthree siloxanehigher acidic bondsdimensional conditions around are the products Si sequentially center ear (003).lier in hydrolyzed the reaction to. The three reaction hydroxyl schematics groups shown (030), for whic h may trialkoxysilanesubsequently precursorscondense to that produce yield three ORMOSIL siloxane and bonds F-ORMOSIL around the are Si center also descriptive(003). of tetraalkoxysilanes,On a given Si atom, e.g., x(400) indicates. While the the number same rules of alkoxy for acid (-OR) and leaving base catalysis groups present, apply, there y the are number some ofkey (-OH) distinct groups,ions. and z the number of (-O-Si) groups, as depicted in the schematics. Thus, for a trialkoxysilaneOn a given under Si atom, acid catalysis, x indic theates precursor the number (300) follows of alkoxy a stepwise (-OR) hydrolysis leaving to form groups (210) present, y the numberThe of sol(-OH)-gel reactionsgroups, andand final z the products number formed of (- Ofrom-Si) alkylgroups- and, as fluoro depicted-trialkoxy insilanes the schematics. are Thus, todistinct (120), andfrom (030) tetraalkoxysilanes products. Under. First, basic and conditions, most notably, condensation the chemical pathways cross-linking are promoted, capacity yielding to form forhigher siloxanea trialkoxysilane dimensional (Si-O-Si) bonds products under is reduced. earlier acid incatalysis Thus, the reaction. for ,complet the The precure reaction condensationsor schematics (300) (003) follows shown, the chemical a for stepwise trialkoxysilane structure hydrolysis is to form precursors that yield ORMOSIL and F-ORMOSIL are also descriptive of tetraalkoxysilanes, e.g., (400). (210)R’SiO to 1.5(120),, referred and to (030) as a silsesquioxane,products. Under in contrast basic toconditions, the SiO2 chemistry condensation for a fully pathways condensed are promoted, yieldingWhiletetraorthosilicate the higher same rules dimensional (004) for. acid Second, and products base hydrophobic catalysis ear apply, lier interactions in there the are reaction (Si some-R’ key ~ . ‘R distinctions.The-Si) reaction influence schematicsthe silane shown for The sol-gel reactions and final products formed from alkyl- and fluoro-trialkoxysilanes are trialkoxysilanesolubility and physical precursors stabilization that that yield is highly ORMOSIL dependent and on solvent F-ORMOSIL, which in turn are influences also descriptive of distinctsol-gel from kinetics tetraalkoxysilanes.. For example, in First, aqueous and most solution notably,, the thefully chemical hydrolyzed cross-linking (030) product capacity exhibits to form an tetraalkoxysilanes, e.g., (400). While the same rules for acid and base catalysis apply, there are some siloxaneamphiphilic (Si-O-Si) character bonds, which is reduced., depending Thus, on for R’ complete chain length, condensation may promot (003),e self the-assembly chemical structurethat is not keyispossible distinct R’SiO1.5 for,ions referred tetraalkoxysilanes. to as a silsesquioxane, [12,13]. Third, in contrastthe steric to and the electronic SiO2 chemistry properties for aof fully the R’ condensed group on tetraorthosilicatetheThe silicon sol - atomgel (004). reactionsinfluence Second, nucleophilicand hydrophobic final products substitution. interactions formed Both (Si-R’ hydrolysis ~f ‘R-Si)rom influencealkyl and- condensationand the silane fluoro solubility - reactiontrialkoxy silanes are distinctandrates physical willfrom be tetraalkoxysilanes stabilization reduced if the that R’ is group highly. First, restrictsdependent and most access on notably, to solvent, the Si the atom, which chemical thus in turn hindering cross influences- linking nucleophilic sol-gel capacity to form siloxane (Si-O-Si) bonds is reduced. Thus, for complete condensation (003), the chemical structure is R’SiO1.5, referred to as a silsesquioxane, in contrast to the SiO2 chemistry for a fully condensed tetraorthosilicate (004). Second, hydrophobic interactions (Si-R’ ~ ‘R-Si) influence the silane solubility and physical stabilization that is highly dependent on solvent, which in turn influences sol-gel kinetics. For example, in aqueous solution, the fully hydrolyzed (030) product exhibits an amphiphilic character, which, depending on R’ chain length, may promote self-assembly that is not possible for tetraalkoxysilanes [12,13]. Third, the steric and electronic properties of the R’ group on the silicon atom influence nucleophilic substitution. Both hydrolysis and condensation reaction rates will be reduced if the R’ group restricts access to the Si atom, thus hindering nucleophilic

Molecules 2019, 24, 2931 3 of 14 kinetics. For example, in aqueous solution, the fully hydrolyzed (030) product exhibits an amphiphilic character, which, depending on R’ chain length, may promote self-assembly that is not possible for tetraalkoxysilanes [12,13]. Third, the steric and electronic properties of the R’ group on the silicon atom influence nucleophilic substitution. Both hydrolysis and condensation reaction rates will be reduced if the R’ group restricts access to the Si atom, thus hindering nucleophilic attack by water (reaction 1) and silanol (reactions 2 and 3). However, through inductive effects, the R’ group can stabilize partial and formal charges that may exist in transition states (TS) during these same nucleophilic substitution reactions. When the R’ group contains electron-withdrawing fluorines, a negatively charged TS would be stabilized as the charge is distributed away from the reaction center. In contrast, an electron providing an R’ group accelerates silane hydrolysis under acidic conditions, where a positively charged TS may arise, but slows hydrolysis under base catalysis due to the anticipated negatively charged TS [14]. Thus, while both fluoro- and alkyl-R’ groups are employed to introduce a hydrophobic character into the resulting condensation product, the electron donating ability of an alkyl R’ group vs. the electron withdrawing nature of a fluoro-containing R’ group lead to differences in reaction rates, which in turn may affect the sol-gel kinetics and thus the size and polydispersity of the condensation products, as well as the porosity and surface area of gels. In the present work, ORMOSIL and F-ORMOSIL sol-gel kinetics are investigated at acidic pH in water without a cosolvent, thus creating an interface between the neat silane and the aqueous reaction solution. The macro silane/water interface is used here to track the hydrolysis and condensation of the ORMOSIL, n-propyltrimethoxy silane (nPM), and the F-ORMSIL analog, 3,3,3-trifluoropropyl trimethoxy silane (3F), as illustrated in Figure2. During hydrolysis, the solubility balance is tipped as the hydrophobic alkoxy leaving groups are replaced with silanol moieties, thus depleting the neat silane that resides as an insoluble phase on top of the water for nPM, and at the bottom of the reaction vessel for the denser 3F. This two-phase reaction system permits in-situ light transmission analysis of hydrolysis and condensation, using turbidity scanning to monitor transmitted and reflected light at defined height increments in the reaction tube. Hydrolysis is reported as an increase in transmission through depletion of the oily lens of neat silane precursor. Condensation correlates to a decrease in transmission as a cloudy sol is formed. Condensation is further confirmed in-situ using dynamic light scattering. Distinct reaction kinetics and assembly mechanisms are observed for nPM and 3F that are attributed to differences in steric and electronic properties of the respective alkyl- or fluoro-groups attached to the silicon atom. At the investigated pH values of 1.7, 2, 3, and 4, these techniques reveal a faster hydrolysis for nPM than 3F, with an opposite trend observed during condensation at the same pH. Molecules 2019, 24, x FOR PEER REVIEW 3 of 15

attack by water (reaction 1) and silanol (reactions 2 and 3). However, through inductive effects, the R’ group can stabilize partial and formal charges that may exist in transition states (TS) during these same nucleophilic substitution reactions. When the R’ group contains electron-withdrawing fluorines, a negatively charged TS would be stabilized as the charge is distributed away from the reaction center. In contrast, an electron providing an R’ group accelerates silane hydrolysis under acidic conditions, where a positively charged TS may arise, but slows hydrolysis under base catalysis due to the anticipated negatively charged TS [14]. Thus, while both fluoro- and alkyl-R’ groups are employed to introduce a hydrophobic character into the resulting condensation product, the electron donating ability of an alkyl R’ group vs. the electron withdrawing nature of a fluoro- containing R’ group lead to differences in reaction rates, which in turn may affect the sol-gel kinetics and thus the size and polydispersity of the condensation products, as well as the porosity and surface area of gels. In the present work, ORMOSIL and F-ORMOSIL sol-gel kinetics are investigated at acidic pH in water without a cosolvent, thus creating an interface between the neat silane and the aqueous reaction solution. The macro silane/water interface is used here to track the hydrolysis and condensation of the ORMOSIL, n-propyltrimethoxy silane (nPM), and the F-ORMSIL analog, 3,3,3- trifluoropropyl trimethoxy silane (3F), as illustrated in Figure 2. During hydrolysis, the solubility balance is tipped as the hydrophobic alkoxy leaving groups are replaced with silanol moieties, thus depleting the neat silane that resides as an insoluble phase on top of the water for nPM, and at the bottom of the reaction vessel for the denser 3F. This two-phase reaction system permits in-situ light transmission analysis of hydrolysis and condensation, using turbidity scanning to monitor transmitted and reflected light at defined height increments in the reaction tube. Hydrolysis is reported as an increase in transmission through depletion of the oily lens of neat silane precursor. Condensation correlates to a decrease in transmission as a cloudy sol is formed. Condensation is further confirmed in-situ using dynamic light scattering. Distinct reaction kinetics and assembly mechanisms are observed for nPM and 3F that are attributed to differences in steric and electronic properties of the respective alkyl- or fluoro-groups attached to the silicon atom. At the investigated MoleculespH2019 values, 24, 2931of 1.7, 2, 3, and 4, these techniques reveal a faster hydrolysis for nPM than 3F,4 ofwith 14 an opposite trend observed during condensation at the same pH.

Figure 2. (A) Schematic of n-propyltrimethoxy silane (nPM) and 3,3,3-trifluoropropyl trimethoxy silane (3F) silaneFigure sol-gel 2. (A) reaction Schematic in aqueous of n-propyltrimethoxy solution catalyzed silane by weak(nPM) acid. and (3,3,3B) Turbiscan-trifluoropropyl height profile trimethoxy of reactionsilane tube(3F) silane monitors sol-gel the reaction depletion in ofaqueous neat silane solution through catalyzed hydrolysis by weak and acid. subsequent (B) Turbiscan particle height formationprofile (sol) of reaction through tubecondensation-induced monitors the depletion turbidity. of neat Reaction silane conditions through hydrolysis can be tuned and to subsequent favor completionparticle of formation hydrolysis (sol) prior through to the onset condensation of condensation.-induced Continuous turbidity. Reaction scanning conditions of the reaction can tubebe tuned providesto favor transmission completion changes of hydrolysis at defined prior height to and the time onset intervals. of condensation. Continuous scanning of the reaction tube provides transmission changes at defined height and time intervals. 2. Results and Discussion

The depletion of the immiscible silane precursor and evolution of a turbid sol for nPM and 3F can be visually assessed through snapshots of reaction vials at defined timepoints as shown in Figure3. The neat silanes are immiscible in water and form visible lenses at the top (nPM) and bottom (3F) of the reaction tubes. The depletion of the lenses as hydrolysis occurs is difficult to discern with the eye, and the boundaries between neat silane and bulk solution are highlighted in the photos. The turbidity associated with condensation is clearly seen, and the expected trend of an increased rate of hydrolysis and condensation is observed with decreasing pH. Acid-catalyzed hydrolysis occurs through a protonation step of the alkoxide group, yielding Si-(ORH)+, promoting nucleophilic attack by water on silicon and forming a pentavalent transition state with the SN2 character. In ORMOSILs, R’Si(OR)3, the electron donating R’ group, stabilizes the positive charge character of the transition state, and thus accelerates hydrolysis relative to tetraorthosilicates (Si(OR)4)[14]. With each hydrolysis step, the transition state is destabilized through the greater electron withdrawing nature of -OH compared to -OR. Similarly, the electron withdrawing nature of the trifluoro moiety in the 3F silane destabilizes the transition state, thus acid-catalyzed 3F hydrolysis is expected to be slower than nPM hydrolysis, as discernible in Figure3. This is most evident for the nPM pH 2.5 vials at the 3 h reaction time, where the lens of neat silane has disappeared completely, whereas at the same pH, the 3F lens is still present at 3 h. To precisely measure hydrolysis and condensation, the optical density, or turbidity, of the solution can be measured at defined heights. A conventional spectrophotometer measures at a fixed height; however, the Turbiscan instrument measures light transmission (and reflection) at defined increments throughout the full height of the tubes, as illustrated schematically in Figure2B[ 15] This allows for a near simultaneous assessment of hydrolysis (lens depletion) and condensation (turbidity evolution) with a resolution of several seconds per scan. In Figure4, select transmission profiles produced for nPM at pH 1.7 and 3.0 are provided to illustrate the procedure. The neat silanes are transparent and transmit incident light; however, where the silane meets the water, the incident light is scattered due to formation of a curved lens at this interface, thus transmission is greatly reduced at this solubility Molecules 2019, 24, x FOR PEER REVIEW 4 of 15 Molecules 2019, 24, 2931 5 of 14 2. Results and Discussion boundary.The In depletion bulk solution, of the the immiscible hydrolyzed silane silanes precursor have and no influence evolution on of lighta turbid transmission, sol for nPM and and only 3F uponcan polycondensation be visually assessed and particlethrough formation snapshots does of reaction light scattering vials at reduce defined transmission timepoints as through shown the in bulk.Figure Thus, 3. Turbiscan The neat measures silanes are hydrolysis immiscible through in water the and retreat form of visible the reaction lenses atinterface the top as (nPM) insoluble and neatbottom silane is (3F) converted of the reaction to soluble tubes. hydrolyzed The depletion silane of while the lenses also measuring as hydrolysis polycondensation occurs is difficult and to phasediscern separation with the through eye, and the the evolution boundaries of scattering between inneat the silane bulk. and This bulk is a solution unique applicationare highlighted of thein instrumentthe photos. for sol-gel The turbidity reaction associated chemistry, with allowing condensation for relative is clearly reaction seen rate, and comparisons. the expected trend of an increased rate of hydrolysis and condensation is observed with decreasing pH.

FigureFigure 3. Time-lapse 3. Time-lapse observation observation of nPM of (0.4 nPM M) (0.4 (left M)images) (left andimages) 3F (0.4 and M) 3F (right (0.4 images) M) (right hydrolysis images) and condensationhydrolysis and at condensation pH 1.7, 2.0, 2.5,at pH and 1.7, 3.0. 2.0, Equal 2.5, and volumes 3.0. Equal of thevolumes two silanes of the two were silanes used, were with used, nPM ﬂoatingwith on nPM top floating of the aqueous on top of solution the aqueous and 3F solution dropping and to 3F the dropping bottom. to Hydrolysis the bottom. depletes Hydrolysis the neatdepletes silane lenses, the neat followed silane by lenses, onset followed of condensation by onset and of condensation turbidity due and to light turbidity scattering due to by light the sol. Aggregationscattering by ofthe the sol. particles Aggregation in the of solthe clears particles the in solution the sol asclears particles the solution drop to as the particles bottom drop of the to the bottom of the reaction tubes. reaction tubes.

The TurbiscanAcid-catalyzed data inhydrolysis Figure4 reveal occurs that through at pH 1.7,a protonation nPM condensation step of the begins alkoxide by 66 group, min, indicated yielding + by aSi uniform-(ORH) drop, promoting in transmissivity nucleophilic throughout attack by the water tube, on yet silicon hydrolysis and forming is not completea pentavalent as indicated transition by state with the SN2 character. In ORMOSILs, R’Si(OR)3, the electron donating R’ group, stabilizes the a near zero transmissivity at the top (30–35 mm) of the tube due to scattering at the neat silane/solution positive charge character of the transition state, and thus accelerates hydrolysis relative to interface. The condensation rate is rapid, forming a completely opaque suspension within 20 min after tetraorthosilicates (Si(OR)4) [14]. With each hydrolysis step, the transition state is destabilized onset as indicated by the zero light transmission at 86 min. In contrast, at pH 3.0, nPM hydrolysis through the greater electron withdrawing nature of -OH compared to -OR. Similarly, the electron required over 500 min to reach completion in which the lens dissolved, and transmission was even withdrawing nature of the trifluoro moiety in the 3F silane destabilizes the transition state, thus throughout the entire tube. At this less acidic pH, the hydrolyzed silane is also very stable, with onset of condensation delayed until after 700 min, and maximum turbidity of the sol obtained after 916 min. Particle aggregation and settling processes are also readily discerned from the scans as an increase in Molecules 2019, 24, 2931 6 of 14 transmission at later times. This is evident for nPM at pH 3.0, where a “wave” of clearing starts at the top of the tube at 916 min and moves down the tube.

FigureFigure 4. Turbidity 4. Turbidity scanning scanning of of the the sol-gel sol-gel reaction reaction forfor nPM in in water water at atpH pH 1.7 1.7 (a) (anda) and pH pH3.0 ( 3.0b). (b). The graphs show light transmission vs. tube height, with 0 mm representing the bottom of the The graphs show light transmission vs. tube height, with 0 mm representing the bottom of the reaction reaction tube. The lens of neat silane occupies the top ~ 5 mm of the 35 mm tube height, causing the tube. The lens of neat silane occupies the top ~5 mm of the 35 mm tube height, causing the dip in dip in transmission seen at early times near the top of the tube. Hydrolysis solubilizes the silane, transmission seen at early times near the top of the tube. Hydrolysis solubilizes the silane, and the thickness of the lens narrows with time. Condensation in the bulk is observed by the drop in light transmissionMolecules 2019, through24, x; doi: FOR the entirePEER REVIEW length of the tube. www.mdpi.com/journal/molecules Molecules 2019, 24, 2931 7 of 14

Animations of the collected data over all times are provided in the Supporting Information files, allowing the hydrolysis, condensation, and aggregation/settling processes to be clearly visualized and contrasted for pH 1.7, 2.0, 3.0, and 4.0. (Videos S1, S2, S3, and S4 in Supplementary Materials) Evidence of a secondary particle growth process is seen in the pH 2.0 transmission data animation, where following an initial condensation and particle settling process, turbidity increases uniformly in the bulk, Moleculesfollowed 201 by9, 2 settling.4, x FOR PEER These REVIEW processes are only observed by scanning the entire reaction tube height,7 of an15 advantage afforded by using the Turbiscan instrument in contrast to a standard spectrophotometer. ByBy selecting selecting the transmission transmission at at a a fixed fixed height height near near the the top top of the of nPMthe nPM lens (e.g., lens (e.g.at a, tube at a height tube heightof 34 mm) of 34 to mm) be monitored, to be monitored, the kinetics the ofkinetics the sol-gel of the processes sol-gel processes can be compared, can be compared as shown, inas Figureshown5. inHowever, Figure 5. the However, onset of the condensation onset of condensation prior to complete prior hydrolysisto complete (e.g., hydrolysis pH 1.7) cannot(e.g., pH be 1.7) detected cannot by bemonitoring detected at by a monitoringfixed height. at Once a fixed the lens height. is dissolved Once the through lens is full dissolved hydrolysis, through condensation-induced full hydrolysis, condensationturbidity occurs-induced uniformly turbidity throughout occurs the uniformly tube. The throughout kinetics for thenPM tube. hydrolysis The kinetics and condensation for nPM hydrolysisfor nPM at and the condensation four pH values for nPM provide at the information four pH values through provide the time information of onset through of the respective the time ofhydrolysis onset of the and respective condensation hydrolysis events, and as condensation well as the slopesevents, for as well each as process. the slopes Figure for each5 shows process. the FigureTurbiscan 5 shows transmission the Turbiscan data thattransmission reflect the data hydrolysis that reflect and the condensation hydrolysis and kinetic condensation profiles of nPMkinetic at profilespH 1.7, of 2.0, nPM 3.0, at and pH 4.0. 1.7, The2.0, transmissivity3.0, and 4.0. The was trans measuredmissivity at was a fixed measured height at of a 34fixed mm height in the of tube, 34 mmwhere in the nPMtube, lenswhere resulted the nPM in alens minimum resulted transmission in a minimum value. transmission At pH 1.7, value rapid. At hydrolysis pH 1.7, rapid of the hydrolysislens eliminated of the scattering lens eliminated and increased scattering transmissivity and increased to 100% transmissivity within 45 min, to followed100% within by an 4 equally5 min, followedrapid condensation-induced by an equally rapid transmissivitycondensation-induced drop to transmissivity 2% after 80 min. drop This to is2% expected after 80 basedmin. This on the is expected2 h snapshot based of the on pH the 1.7 2 h reaction snapshot tube of in the Figure pH 3 1.7. By reaction 900 min, tube the transmissivity in Figure 3. By increased 900 min to, 50%the tasransmissivity condensed particlesincreased aggregated to 50% as and condensed settled fromparticles the solution,aggregated which and can settled also from be seen the in solution the 24 h, whichsnapshot can ofalso nPM be atseen pH in 1.7 the in 24 Figure h snapshot3. of nPM at pH 1.7 in Figure 3.

FigureFigure 5. HydrolysisHydrolysis and and condensation condensation kinetics kinetics of nPM of (0.4 nPM M) (0.4 at pH M) 1.7, at 2.0, pH 3.0, 1.7, and 2.0, 4.0. Transmissivity 3.0, and 4.0. Transmissivitydata were taken data at the were height taken where at the transmissivity height where is transmissivity the lowest at the is the beginning lowest dueat the to beginning the presence due of tonPM. the The presence right panel of nPM. shows The the right transmission panel shows during thethe transmission ﬁrst 450 min, during with putative the first (xyz) 450 min hydrolysis, with putativeand condensation (xyz) hydrolysis process and nomenclature condensation according process tonomenclature Figure1. according to Figure 1.

IncreasingIncreasing the the pH pH above above 1.7 1.7 (e.g. (e.g.,, 2.0 2.0,, 3.0, 3.0, or or4.0) 4.0) delays delays the theonset onset of hydrolysis of hydrolysis,, decreases decreases the hydrolysisthe hydrolysis rate rate(lower (lower initial initial slopes), slopes), and and extends extends the the stability stability of of the the hydrolyzed hydrolyzed product product (high (high transmissivitytransmissivity plateau) plateau) as as seen seen in in Figure Figure 55.. These data demonstrate turbidity scanning as a facile meansmeans to to evaluate evaluate the the key key sol sol-gel-gel steps steps of: of: 1) 1) N Neateat silane silane hydrolysis hydrolysis,, 2) 2) hydrolysis hydrolysis product product stability stability,, followedfollowed by 3)3) condensationcondensation and and 4) 4) subsequent subsequent particle particle aggregation, aggregation settling,, settling, and resuspension and resuspension events. eventsThe transmissivity. The transmissivity patterns patterns along thealong tube the length tube length during during the hydrolysis the hydrolysis and condensation and condensation of nPM of nPMare better are better visualized visualized in the in zoomed-inthe zoomed analysis-in analysis ofthe of the ﬁrst first 450 450 min min of theof the transmissivity transmissivity curves curves in inFigure Figure5. Under5. Under acid acid catalysis, catalysis, ORMOSILs, ORMOSILs such, such as nPM, as nPM undergo, undergo a stepwise a stepwise hydrolysis hydrolysis process process from from(300) (300)to (210), to (120),(210), and(120), ﬁnally and (030)finally [16 (030–18].) [16 With–18] each. With step, each silane step solubility, silane solubilityincreases, increases, leading to leading to near 100% transmissivity, as seen for the nPM data. The expected influence of pH on nPM hydrolysis is seen in these data, with hydrolysis complete by 50 min at pH 1.7, 100 min at pH 2.0, 200 min at pH 3, and 350 min at pH 4.0. Likewise, hydrolyzed nPM stability is highly sensitive to reaction pH, increasing from ~10 min stability at pH 2.0 to over 24 h at pH 4.0. Aggregation of the condensed species leads to particle settling and a further increase in transmissivity, which is observed at each of the investigated pH values. Silanes that are denser than water, such as 3F, can also be measured using Turbiscan, as shown in Figure 6 for the full tube height transmissivity scans, which report hydrolysis and condensation profiles of 3F at pH 1.7 and pH 3.0. As seen in Figure 6, 100% transmissivity is observed throughout

Molecules 2019, 24, 2931 8 of 14 near 100% transmissivity, as seen for the nPM data. The expected influence of pH on nPM hydrolysis is seen in these data, with hydrolysis complete by 50 min at pH 1.7, 100 min at pH 2.0, 200 min at pH 3, and 350 min at pH 4.0. Likewise, hydrolyzed nPM stability is highly sensitive to reaction pH, increasing from ~10 min stability at pH 2.0 to over 24 h at pH 4.0. Aggregation of the condensed species leads to particle settling and a further increase in transmissivity, which is observed at each of the investigated pH values. Silanes that are denser than water, such as 3F, can also be measured using Turbiscan, as shown in Figure6 for the full tube height transmissivity scans, which report hydrolysis and condensation profiles of 3F at pH 1.7 and pH 3.0. As seen in Figure6, 100% transmissivity is observed throughout the solution except at the tube bottom, where neat 3F created a 2 mm thick lens that scattered the incident light, thus reducing transmitted light. As with nPM, 3F also shows an increase in hydrolysis and condensation reaction rates with decreasing pH that are evident in these snapshots of the transmission scans. The full set of scans can be viewed as an animation in Video S5, S6, S7, and S8, more clearly revealing secondary processes of condensation product aggregation and settling as reported by drops in turbidity throughout the tube. Following decreases in turbidity indicating particle settling, 3F, like nPM, shows subsequent turbidity increases, which may reflect secondary nucleation and growth events, or resuspension of aggregates that separate due to charge repulsion. By monitoring the transmission at a fixed height near the bottom of the 3F lens (e.g., 6 mm from bottom), where lowest transmissivity appears, the kinetics of the sol-gel processes can be compared at the different pH values as shown in Figure7. The lens curvature leads to light scattering and reduced transmissivity; where the lens meets the tube wall, it forms a cylinder and the transmissivity therefore increases again below this curved interface of the lens with the water. At pH 1.7, the transmissivity measured at this fixed height increased from 5% at the start of the reaction to about 65% at 30 min, followed by a decrease in transmission. In contrast to nPM, maximum transmissivity was never more than about 65% for 3F at pH 1.7, indicating that under these conditions, 3F was likely not fully hydrolyzed prior to the onset of condensation; however, at higher pH (e.g., 3.0 or 4.0), stable hydrolyzed 3F species were attained. The hydrolyzed 3F lifetime is greatest at pH 3.0, unlike the hydrolyzed nPM, which is highest at pH 4.0. Also, the shortest hydrolyzed 3F lifetime is at pH 2, whereas it is at pH 1.7 for hydrolyzed nPM. Based on the findings from the Turbiscan analysis as discussed, a brief summary of the optimum hydrolysis, condensation, sedimental time, and the stability period of the hydrolysis state is presented in Table1. 3F is more acidic than nPM and hydrolyzed faster. For both silanes, the data show that the lifetime of the anticipated hydrolyzed species (high transmissivity plateau) is increased as pH is increased. Under mildly acidic conditions, hydrolyzed silanes are stable in a manner highly dependent on the side group. For bulky R’ groups, such as a phenyl ring, the silanol stability is enhanced due to steric hindrance against nucleophilic substitution [19]. For base-catalyzed sol-gel reactions, both hydrolysis and condensation occur in parallel, thus condensation occurs via the alcohol-liberating condensation pathway, as depicted in reaction (2). The selection of acid or base catalysts influences the resulting condensation and gel formation, with more porous gels resulting for base-catalyzed pathways and denser gels for acid catalyzed pathways [20]. Molecules 2019, 24, x FOR PEER REVIEW 2 of 2

and the thickness of the lens narrows with time. Condensation in the bulk is observed by the drop in lightMolecules transmission2019, 24, 2931 through the entire length of the tube. 9 of 14

Figure 6.Figure Turbidity 6. Turbidity scanning scanning of the of thesol-gel sol-gel reac reactiontion for for 3F 3F in water at at pH pH 1.7 1.7 (a )(a and) and pH pH 3.0 (3.0b). (b). The graphs Theshow graphs light show transmission light transmission vs. tube vs. tube height, height, with with 0 0 mmmm representing representing the bottomthe bottom of the reaction of the reaction tube. Thetube. lens The of lens neat of silane neat silane occupies occupies the the bottom bottom ~2 ~2 mm mm ofof the the 35 35 mm mm tube tube height, height, causing causing the dip the dip in in transmission seen at early times near the bottom of the tube. Hydrolysis solubilizes the silane, transmission seen at early times near the bottom of the tube. Hydrolysis solubilizes the silane, and and the lens thickness narrows with time. Condensation in the bulk is observed by the drop in light the lenstransmission thickness initiallynarrows at thewith bottom time. and Condensation later through the in entire the length bulk ofis the observed tube. by the drop in light transmission initially at the bottom and later through the entire length of the tube.

Molecules 2019, 24, x FOR PEER REVIEW 10 of 15

By monitoring the transmission at a fixed height near the bottom of the 3F lens (e.g., 6 mm from bottom), where lowest transmissivity appears, the kinetics of the sol-gel processes can be compared at the different pH values as shown in Figure 7. The lens curvature leads to light scattering and reduced transmissivity; where the lens meets the tube wall, it forms a cylinder and the transmissivity therefore increases again below this curved interface of the lens with the water. At pH 1.7, the transmissivity measured at this fixed height increased from 5% at the start of the reaction to about 65% at 30 min, followed by a decrease in transmission. In contrast to nPM, maximum transmissivity was never more than about 65% for 3F at pH 1.7, indicating that under these conditions, 3F was likely not fully hydrolyzed prior to the onset of condensation; however, at higher pH (e.g., 3.0 or 4.0), stable hydrolyzed 3F species were attained. The hydrolyzed 3F lifetime is greatest at pH 3.0, unlike the hydrolyzed nPM, which is highest at pH 4.0. Also, the shortest hydrolyzed 3F lifetime is at pH 2, whereas it is at pH 1.7 for hydrolyzed nPM. Based on the findings from the Turbiscan analysis as discussed, a brief summary of the optimum hydrolysis, condensation, sedimental time, and the stability period of the hydrolysis state is presented in Table

Molecules1. 2019, 24, 2931 10 of 14

FigureFigure 7. 7. HydrolysisHydrolysis and and condensation condensation kinetics kinetics of of 3F 3F (0.4 (0.4 M) M) based based on on transmissivity, transmissivity, using using Turbiscan-analyzedTurbiscan-analyzed data data at at pH pH 1.7, 1.7, 2.0, 2.0, 3.0, 3.0, and and 4.0. 4.0. Transmissivity Transmissivity data data for for di differentﬀerent pH pH are are taken taken at at thethe height height where where transmissivity transmissivity is is the the lowest lowest at at the the beginning beginning due due to to the the presence presence of of 3F. 3F. A A zoomed zoomed ﬁgurefigure in in the the right right shows shows how how the the chemical chemical transformation transformation occurred occurred in in 3F 3F during during the the hydrolysis hydrolysis and and condensationcondensation process. process. Table 1. Summary of the average hydrolysis, condensation, sedimental time, and stability of the

hydrolyzed state at pH 1.7, 2.0, 3.0, and 4.0 of nPM and 3F.

Average Average Average Condensation Stability of pH Condensation Sedimentation Hydrolysis Time Starting Time Hydrolyzed State Time Time

min min min min min 1.7 45 46 1 100 1000

nPM 2.0 100 110 10 200 1000 3.0 200 700 500 900 1100 4.0 350 1850 1500 2800 >4000 1.7 30 30 0 120 250 3F 2.0 80 80 0 90 275 3.0 180 600 420 >800 NA 4.0 275 500 225 >800 NA

The pH trends for 3F condensation rates are also distinct as compared to nPM. The 3F condensation rate is faster at pH 2 than pH 1.7, and the condensation rate at pH 4.0 is faster than that of pH 3.0. These distinctions in the nPM and 3F condensation trends at the investigated pH values are schematically depicted in Figure8. The distinctions in the sol-gel kinetics are attributable to the electron withdrawing and donating nature of the fluoro- and organo-side chains of 3F and nPM, respectively. Inductive effects can significantly shift the pKa of acids. Distinct isoelectric points of ORMOSIL and F-ORMOSIL analogs, such as nPM and 3F, are expected. As reviewed by Coltrain and Kelts, silane condensation is specifically acid and base catalyzed: Above the silica isoelectric point of ~pH 2.5, condensation proceeds by nucleophilic attack of deprotonated silanols on neutral silicates, while below the isoelectric point, the reaction proceeds by protonation of silanols followed by electrophilic attack [21]. Molecules 2019, 24, x FOR PEER REVIEW 11 of 15

Table 1. Summary of the average hydrolysis, condensation, sedimental time, and stability of the hydrolyzed state at pH 1.7, 2.0, 3.0, and 4.0 of nPM and 3F.

Average Stability of Average Average pH Hydrolysis Condensation Hydrolyzed Condensation Sedimentation Time Starting Time State Time Time min min min min min 1.7 45 46 1 100 1000 nPM 2.0 100 110 10 200 1000 3.0 200 700 500 900 1100 4.0 350 1850 1500 2800 > 4000 1.7 30 30 0 120 250 3F 2.0 80 80 0 90 275 3.0 180 600 420 > 800 NA 4.0 275 500 225 > 800 NA

3F is more acidic than nPM and hydrolyzed faster. For both silanes, the data show that the lifetime of the anticipated hydrolyzed species (high transmissivity plateau) is increased as pH is increased. Under mildly acidic conditions, hydrolyzed silanes are stable in a manner highly dependent on the side group. For bulky R’ groups, such as a phenyl ring, the silanol stability is enhanced due to steric hindrance against nucleophilic substitution [19]. For base-catalyzed sol-gel reactions, both hydrolysis and condensation occur in parallel, thus condensation occurs via the alcohol-liberating condensation pathway, as depicted in reaction (2). The selection of acid or base catalysts influences the resulting condensation and gel formation, with more porous gels resulting for base-catalyzed pathways and denser gels for acid catalyzed pathways [20]. The pH trends for 3F condensation rates are also distinct as compared to nPM. The 3F condensation rate is faster at pH 2 than pH 1.7, and the condensation rate at pH 4.0 is faster than that of pH 3.0. These distinctions in the nPM and 3F condensation trends at the investigated pH values are schematically depicted in Figure 8. The distinctions in the sol-gel kinetics are attributable to the electron withdrawing and donating nature of the fluoro- and organo-side chains of 3F and nPM, respectively. Inductive effects can significantly shift the pKa of acids. Distinct isoelectric points of ORMOSIL and F-ORMOSIL analogs, such as nPM and 3F, are expected. As reviewed by Coltrain and Kelts, silane condensation is specifically acid and base catalyzed: Above the silica isoelectric point of ~pH 2.5, condensation proceeds by nucleophilic attack of deprotonated silanols onMolecules neutral2019 , silicates24, 2931 , while below the isoelectric point, the reaction proceeds by protonation11 of of 14 silanols followed by electrophilic attack [21].

Figure 8. pH based condensation trends of nPM and 3F identified from Turbiscan. Figure 8. pH based condensation trends of nPM and 3F identified from Turbiscan. Due to electron-donating inductive effects, the isoelectric point for nPM was expected to be at least 4.5 [22], and our data for nPM agree with acid-specific catalysis of condensation as the pH decreased from 4.0 to 1.7. For 3F, the presence of the highly electronegative fluorines lowers the isoelectric pH relative to the non-fluorinated nPM. TEOS has a point of zero charge near 2.2, and 3F appears to be between this and that of nPM, as the fluorines are three carbons removed from the silicon center, so the electron withdrawing inductive effect is diminished. These trends are depicted in Figure8. To better assess the hydrolysis and condensation kinetics, dynamic light scattering (DLS) was used to monitor colloid evolution during the condensation process at pH 1.7 and 3.0. Figure9a shows a comparative hydrodynamic size of the colloidal particles formed during the hydrolysis and condensation of 3F and nPM at pH 1.7. For nPM, light scattering corresponding to sub-100 nm particle formation was observed within 80 to 90 min. After ~180 min, the nPM particle sizes exceeded 1000 nm and the scattering signal was lost as particles aggregated and settled out of solution. Figure9a shows a similar particle evolution pattern for 3F. The slower hydrolysis and condensation kinetics of 3F compared to nPM at pH 1.7 may reflect the distinct inductive effects shifting the isoelectric points of the silanes in opposite directions [22]. At pH 3.0, evolution of the colloidal particles was delayed compared to that at pH 1.7, as shown in Figure9b; however, at the higher pH, particles formed in the 3F solution before the nPM solution. Also, decreases in particle sizes followed by subsequent increases are observed in Figure9b, which may be due to the sedimentation of larger particles from solution followed by particle remodeling/regrowth phenomena. Both the Turbiscan and DLS data reflect a crossing over of the isoelectric pH of the 3F when the pH increases from 1.7 to 3.0, arising from the electron withdrawing trifluoro-propyl side chain. For the non-fluorinated silane analog, nPM, all investigated pH values are below the isoelectric pH, which increased above 4.0 due to the electron donating character of the n-propyl group. Both nPM and 3F follow the acid-catalyzed pathway depicted in Figure1; however, as visually observed in Figure3, the nPM forms a highly turbid sol more rapidly than 3F due to the electron-donating, positive inductive effect of the n-propyl chain that stabilizes the positive charge on the silicon atom arising during protonation of the alkoxy leaving group. Turbiscan measures silane hydrolysis and condensation through light scattering and absorbance, respectively. Changes in the height of the light scattering event at the curved interface formed by the neat silane with water reflects the solubilization of the silanes upon hydrolysis. As shown for nPM and 3F, this can be employed with silanes that are less dense (nPM) and denser (3F) than water. Condensation leading to reduction in light transmission is simultaneously measured throughout the reaction tube, which is distinct from measuring sol formation in a standard spectrophotometer that measures at a fixed tube height. DLS can also be employed as an in-situ technique for reporting condensation but not hydrolysis. Molecules 2019, 24, x FOR PEER REVIEW 12 of 15

Due to electron-donating inductive effects, the isoelectric point for nPM was expected to be at least 4.5 [22], and our data for nPM agree with acid-specific catalysis of condensation as the pH decreased from 4.0 to 1.7. For 3F, the presence of the highly electronegative fluorines lowers the isoelectric pH relative to the non-fluorinated nPM. TEOS has a point of zero charge near 2.2, and 3F appears to be between this and that of nPM, as the fluorines are three carbons removed from the silicon center, so the electron withdrawing inductive effect is diminished. These trends are depicted in Figure 8. To better assess the hydrolysis and condensation kinetics, dynamic light scattering (DLS) was used to monitor colloid evolution during the condensation process at pH 1.7 and 3.0. Figure 9a shows a comparative hydrodynamic size of the colloidal particles formed during the hydrolysis and condensation of 3F and nPM at pH 1.7. For nPM, light scattering corresponding to sub-100 nm particle formation was observed within 80 to 90 min. After ~180 min, the nPM particle sizes exceeded 1000 nm and the scattering signal was lost as particles aggregated and settled out of solution. Figure 9a shows a similar particle evolution pattern for 3F. The slower hydrolysis and condensation kinetics of 3F compared to nPM at pH 1.7 may reflect the distinct inductive effects shifting the isoelectric points of the silanes in opposite directions [22]. At pH 3.0, evolution of the colloidal particles was delayed compared to that at pH 1.7, as shown in Figure 9b; however, at the higher pH, particles formed in the 3F solution before the nPM solution. Also, decreases in particle sizes followed by subsequent increases are observed in Figure 9b, which may be due to the sedimentation of larger particles from solution followed by particle remodeling/regrowth Molecules 2019, 24, 2931 12 of 14 phenomena.

Figure 9. Comparison of the hydrodynamic sizes of the colloidal particles evolved during the hydrolysis andFigure condensation 9. Comparison of nPM (0.1 of the M) and hydrodynamic 3F (0.1 M) at sizes pH 1.7 of (athe) and colloidal pH 3.0 (b particles). Slopes evolved are indicated during for the a relativehydrolysis comparison and condensation of particle formationof nPM (0.1 rates. M) and 3F (0.1 M) at pH 1.7 (a) and pH 3.0 (b). Slopes are indicated for a relative comparison of particle formation rates. 3. Materials and Methods Both the Turbiscan and DLS data reflect a crossing over of the isoelectric pH of the 3F when the 3.1.pH Source increases of Chemicals from 1.7 to 3.0, arising from the electron withdrawing trifluoro-propyl side chain. For the Double non-fluorinated distilled deionized silane analog, (DI) waternPM, wasall investigated collected from pH Barnstead values areTM belowMegaPure the TMisoelectricGlass Still pH, (Thermowhich incr Scientific);eased above hydrochloric 4.0 due to acid the (HCl)electron 1.0 donating N was purchased character fromof the Ricca n-propyl Chemical group. Company Both nPM (Arlington,and 3F follow TX, USA);the acid 3,3,3-trifluoropropyl-catalyzed pathway trimethoxydepicted in silane Figure (3F, 1; however,>95% purity, as visually MW = observed218.3, d = in 1.14Figure g/mL) 3, andthe nPM n-propyltrimethoxy forms a highly silane turbid (nPM, sol more>95% rapidly purity, than MW 3F= due164.3, to d the= 0.94 electron g/mL)-donating, were purchasedpositive inductive from Gelest, effect Inc. of (Morrisville, the n-propyl PA, chain USA); that and stabilizes potassium the chloride positive (KCl) charge was on purchased the silicon from atom Thermoarising Fisherduring Scientific, protonation Fair of Lawn, the alk NJ,oxy USA. leaving group. Turbiscan measures silane hydrolysis and condensation through light scattering and absorbance, respectively. Changes in the height of the 3.2.light Monitoring scattering Reaction event Time at the through curved the interfaceSilane Phase formed by the neat silane with water reflects the solubilizationFour different of the 0.4 Msilanes solutions upon were hydrolysis. prepared As for shown both nPM for nPM and 3Fand at 3F, pH this 1.7, can 2.0, 2.5,be employed and 3.0 using with 20,silanes 10, 5, andthat 1are mM less HCl dense solution (nPM) respectively. and denser To (3F) prepare than 0.4 water. M nPM Condensation and 3F solution, leading 2.32 to mL reduction acidic DI in water was pipetted into a glass test tube (0.5 cm2 inner surface area) and then 0.18 mL nPM or 3F was poured in it. The hydrolysis and condensation were monitored using a Nikon COOLPIX P510 camera.

3.3. Turbiscan Analysis Hydrolysis and condensation behavior of nPM (0.4 M) and 3F (0.4 M) at pH 1.7, 2.0, 3.0, and 4.0 were followed by measuring the transmissivity of light (850 nm) through the samples using a Turbiscan instrument (Turbiscan Classic, MA200, L’Union, France), and data were analyzed using Turbisoft 1.2.1. All solutions were prepared as described in the previous paragraph, and the pH 4.0 solution was 0.1 mM HCl. All solutions were placed in the Turbiscan tubes to a height of 35 0.5 mm, and transmissivity ± was measured as a function of time. Transmissivity was measured throughout the height of the solution in the tubes for 12 to 96 h, and measurements were taken at 2 to 22 min intervals, depending on the pH of the solutions, which inﬂuenced reaction rates. Data were plotted using Sigma Plot 10.0. Videos of the Turbiscan-obtained data were prepared using MATLAB R2018b.

3.4. Dynamic Light Scattering (DLS) Analysis Evolution of polymeric NPs during nPM and 3F hydrolysis and condensation were analyzed by DLS (DynaPro NanoStar, Wyatt Technology Corporation, Santa Barbara, CA, USA), with a 658 nm laser. Measurements were taken using a quartz cuvette (JC-426, 1 µL) with 0.7 mL of nPM (0.1 M) and 3F (0.1 M) solution at pH 1.7 and 3.0. The silane concentrations of nPM (0.4 M) and 3F (0.4 M) used in the Turbiscan analysis were too high for the DLS instrument to measure single scattering events. Molecules 2019, 24, 2931 13 of 14

Furthermore, the sol-gel reactions reported through DLS cannot be directly compared with those obtained from turbidity scanning due to diﬀerences in the geometries of the sample tubes and cuvettes used for Turbiscan and DLS, respectively. The data were recorded as an average of 10 ﬁve-second acquisitions. Hydrodynamic radii (Rh) of polymeric NPs were obtained using Dynamic software (version 7.0.3, Wyatt Technology Corporation, Santa Barbara, CA, USA), where the Stokes–Einstein equation was applied to convert the intensity auto-correlation into particle sizes. Inner geometry of the quartz cuvette was circular in shape and the DLS laser light was passed through approximately 1/3 of the height of the cuvette bottom.

4. Conclusions The sol-gel kinetics of a short chain alkyl-silane, nPM, and fluorinated analog, 3F, were investigated in aqueous solutions using turbidity scanning. The immiscibility of the silanes in water creates a reaction interface that scatters incident light, allowing the dissolution of the insoluble silane lens through hydrolysis to be tracked. Simultaneously, the technique tracks condensation by scanning through the height of the tube to monitor condensation product-induced light scattering in the bulk solution. The trifluoro-propyl moiety renders 3F denser than water, creating the reaction interface in the bottom of the reaction tube whereas the non-fluorinated nPM resides on top of the water. For 3F, this creates some potential interference as condensation products aggregate and settle toward the bottom of the tube. The electron-withdrawing fluoro-groups also accelerate the condensation of 3F compared to nPM. The use of optical turbidity scanning and the dynamic light scattering technique showed the distinct pattern of the hydrolysis and condensation kinetics of nPM and 3F.

Supplementary Materials: The following are available online, Video S1: nPM Turbiscan full scan at pH 1.7, Video S2: nPM Turbiscan full scan at pH 2.0, Video S3: nPM Turbiscan full scan at pH 3.0, Video S4: nPM Turbiscan full scan at pH 4.0, Video S5: 3F Turbiscan full scan at pH 1.7, Video S6: 3F Turbiscan full scan at pH 2.0, Video S7: 3F Turbiscan full scan at pH 3.0, and Video S8: 3F Turbiscan full scan at pH 4.0 Author Contributions: The conceptualization, methodology, validation, and formal analysis were conducted by A.B.M.G. and D.W.B.; A.B.M.G. performed the experiments and prepared the original draft, with D.W.B. providing review and editing. D.W.B. secured funding for this research. Funding: This research was funded by the Utah State University Agriculture Experiment Station, Project 1052. Acknowledgments: This research was supported by the Utah Agricultural Experiment Station, Utah State University, and approved as journal paper number #9229. We would like to thank Akhter Mahmud Nafi from the Mechanical and Aerospace Engineering department of USU for his contribution in the Turbiscan kinetic animations. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Sample Availability: Samples of the 3F and nPM are available from the authors.

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